Progressive Multifocal Leukoencephalopathy (PML) is a major life threatening complication in patients with underlying immunosuppressive disorders (e.g. AIDS) and in patients undergoing immunotherapy for autoimmune diseases such as multiple sclerosis, Crohn's disease, severe plaque psoriasis, systemic lupus erythematosis, hematologic malignancies, and rheumatoid arthritis. The disease is paradoxically caused by a common human polyomavirus following activation from a latent to a lytic phase of growth. There are several critical gaps in our understanding of the basic biology of PML. First, the anatomical site of virus latency is not known but kidney, tonsil, bone marrow, and brain have all been postulated to be involved. Second, the mechanisms that govern latency versus lytic growth of the virus are not well understood. Third, the mechanisms of viral spread to the CNS and within the CNS are not known. We hypothesize that virus induced signals reprogram the cellular environment to promote replication and spread and that epigenetics plays a role in governing the balance between latency and activation. The experiments proposed here will define these signals and the molecular pathways involved in viral pathogenesis. These mechanisms and pathways may be amenable to pharmacological intervention.